A seven year old Quarter Horse/Arab mare was evaluated for episodes of rhabdomyolysis after light work. Muscle biopsies revealed variation in fiber size along with pale, granular subsarcolemmal areas which stained strongly positive for glycogen. Clinical signs and histopathologic findings were consistent with those documented by Stephanie Valberg in Quarter Horses and Quarter Horse related breeds with recurrent exertional rhabdomyolysis. An experimental high fat diet was instituted and pre and post exercise serum CK and AST levels are measured after 6 weeks and 10 weeks. Serum muscle enzyme levels remained within normal ranges for all samples.